Conventionally, a collision avoidance device is known (see Patent Document 1 for example). The collision avoidance device calculates a predictive value of a possible collision based on a velocity of a self vehicle and an inter-vehicular distance between the self vehicle and a leading vehicle. Then, the collision avoidance device outputs an alarm if the calculated predictive value is greater than or equal to a predetermined threshold value. This collision avoidance device judges whether a driver of the self vehicle intends to accelerate the self vehicle based on an acceleration of the self vehicle. If the device judges that the driver intends to accelerate the self vehicle, the device adjusts the predictive value downward or adjusts the threshold value upward in order to make the alarm less likely to output. Also, this collision avoidance device judges whether a behavior of the leading vehicle is abnormal based on an acceleration of the leading vehicle. If the device judges that a behavior of the leading vehicle is abnormal, the device adjusts the predictive value upward or adjusts the threshold value downward in accordance with the acceleration of the leading vehicle in order to make the alarm more likely to output.
Vehicle control devices are also known (see Patent Documents 2 and 3 for example). These vehicle control devices calculate a control value based on an inter-vehicle distance between a self vehicle and a leading vehicle, a relative velocity, a relative acceleration, and a velocity of the self vehicle. Then, these devices output an alarm if the calculated control value is greater than or equal to a predetermined threshold value. These devices calculate the control value based on an acceleration of the leading vehicle in place of the relative acceleration if the leading vehicle decelerates relative to the self vehicle. In this way, the devices reflect the acceleration of the leading vehicle on a determination of an alarm output timing independently. Meanwhile, the acceleration of the leading vehicle changes independently of a manipulation of a driver of the self vehicle.
An in-car device is also known (see Patent Document 4 for example). The in-car device outputs an alarm if an inter-vehicular distance between a self vehicle and a leading vehicle is less than a distance calculated based on a relative velocity, a velocity of the self vehicle, an acceleration of the self vehicle, and a degree of certainty that a driver of the self vehicle intends to follow the leading vehicle while shortening the inter-vehicular distance. The degree of certainty is calculated based on standard deviations of each of the inter-vehicular distance, the relative velocity, and the relative acceleration within a predetermined period of time. This in-car device calculates the distance to be shorter if the driver intends to follow the leading vehicle while shortening the inter-vehicular distance, i.e., if the degree of certainty is high, in order to make the alarm less likely to output.    [Patent Document 1] Japanese Patent Publication No. 2005-53384    [Patent Document 2] Japanese Patent Publication No. 2010-274838    [Patent Document 3] Japanese Patent Publication No. 2011-6038    [Patent Document 4] Japanese Patent No. 3733768